Tardigrades Are Still A Complete Evolutionary Mystery

You're probably aware that nature's most badass animal is undoubtedly the tiny tardigrade, or water bear. They might be small, but unlike your weak butt, they can live a life without water, withstand temperatures from -328 to 304 degrees Fahrenheit, and even survive the depths of space. How did evolution make such a strange creature, and who are its relatives?

A team of scientists in the United Kingdom and Japan sequenced one tardigrade species' genome and compared it to another to unlock the animal's secrets, including the genetic basis of its survival skills. But as far as a closest evolutionary relative, the data's still inconclusive.

Tardigrade durability lies in their ability to lose all of their water and curl up into "tuns." Losing the water from cells should be a lethal process, but there's a host of molecules in the tardigrade's cells that seem to prevent the cell death, according to past research published in PLoS One. That paper also reports that certain nematodes and arthropods seem to be able to dry up, too.

Other papers have found difficulty determining what animals the tardigrade may have evolved from and the biological basis for its superpowers, but have identified certain responsible genes, according to a new study published in PLoS Biology. They have also implied that lots of the water bear's genome, possibly a sixth of it, came from horizontal gene transfer, genetic material acquired from other animals, including those of other species.

Water bears, known to scientists as tardigrades, are famously adorable microscopic creatures who can survive anything. freezing, total dehydration, radiation bombardment, and even the vacuum of deep space. Now scientists have sequenced a tardigrade genome, and are very surprised by the results.

So, this team put together a genome for the Hypsibius dujardini tardigrade species from around 900,000 individuals and compared it to the existing genome of the Ramazzottius varieornatus species to see what they could learn.

Aside from differences in the genome sizes (H. dujardini's was much larger), they found further information about the genes that control the proteins that protect the tardigrade's cells, according to New Scientist. On top of that, the amount of horizontal gene transfer seemed much lower than previous studies have suggested, closer to one per cent of their genome. That would take a major confounding factor out of their evolutionary story.

But despite all the work, the scientists still couldn't really tell whether the water bear is more closely related to the nematode, or to arthropods like insects and crustaceans.

"Even the full genomes of two tardigrades, which the authors report here, were not sufficient to decide whether tardigrades were closer to the arthropods or the nematodes," biologist Thorsten Burmester from the University of Hamburg in Germany, who was not involved with the study, told The Scientist in an email.

Of course, this is a single paper and an ongoing story, so more research will naturally shed light on what's really going on.

Science has lost yet another round against the seemingly indefatigable water bear. The tardigrade refuses to be fully understood.

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Comments

Ive always found Tardigrades amazing. A cool "way out" there theory Ive always thought about, what if the Tardigrade never actually evolved "on" earth? Its about the best candidate Ive seen that could "hitch" a ride on a bit of rock from somewhere else.

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